1. Field of the Invention
This invention relates to the measurement of the thickness and uniformity of the layers of a semiconductor wafer employing a sectioning machine. More particularly, the present invention relates to an improved wafer chuck for holdling large wafers for engagement with a rotatable abrasive tool of a sectioning machine.
2. Description of the Prior Art
A major portion of the semiconductor industry is concerned with the manufacture of integrated circuits. Integrated circuits are becoming more dense and a large number of devices are being placed on a single wafer which decreases the cost of manufacture and effectively decreases the power requirements for the individual devices. These more complicated and highly miniaturized devices require precise control of the thickness and uniformity of the layers of material deposited on the substrate wafer to make the semiconductor chips. During various stages of production of these integrated circuit devices it has become necessary to measure the thicknesses of the layers employing sectioning machines.
Early prior art sectioning machines employed a machine tool approach. The semiconductor wafer was clamped onto a rigid flat plate leaving the top surface of the semiconductor wafer exposed. A rotatable abrasive tool was engaged against the top surface of the semiconductor wafer to form a groove therein. It was found that the top of the semiconductor wafer did not uniformly engage the abrasive tool along a line because contaminates such as particles of dirt would get between the back of the wafer and the mounting plate and/or the semiconductor wafer was not uniformly thick or flat so as to cause the top surface of the semiconductor wafer to be out of parallel alignment with the abrasive tool.
My U.S. Pat. No. 3,745,716 issued July 17, 1973 discloses a sectioning machine in which a semiconductor wafer is mounted in a mechanical wafer chuck. The wafer chuck is provided with flat jaw reference surfaces which engage the front polished surface of the wafer. A force is applied to the rear of the semiconductor wafer to force the front face of the wafer into flat engagement with the jaw reference surface. It was found that the wafer chuck of this sectioning machine was expensive to manufacture since the jaw reference surfaces were ground and precision finished from a block of stainless steel. When the semiconductor wafer is engaged with the jaw reference surfaces the wafer becomes part of the rigid wafer chuck and precise alignment of the chuck relative to the rotational axis of the abrasive tool is required. Such precise alignment is difficult and expensive to provide.
The state of the semiconductor wafer art has been advanced to a point where much larger diameter semiconductor wafers are being used. These larger wafers are more uniform in thickness and flatness than smaller wafers produced in the prior art. These larger wafers require much larger chucks. When the larger wafers are mounted on larger chucks it is desirable to either improve the accuracy or maintain the accuracy of the sectioning machines used heretofore. Since the diameter of the wafer being sectioned is much larger, the wafer chuck which holds the semiconductor wafer would be expected to be heavier and more rugged to achieve greater stability and accuracy over a greater distance. It was found that the wafer chucks heretofore used with sectioning machines for smaller wafers were excessively expensive to manufacture for use with larger semiconductor wafers.
It would be desirable to provide a wafer chuck for a sectioning machine which is less expensive to manufacture than wafer chucks known heretofore and which provides extremely precise line sections when employed for use with either large or small semiconductor wafers.